1. Field of the Invention
The present invention is directed to methods of making a thermoplastic composite material that includes a cellulosic material and an anhydride containing copolymer and to articles made from composite materials made according to the method.
2. Description of the Prior Art
Traditionally, fences and decks have been made of components fashioned from solid wood. Wood fences and decks are often considered more aesthetically appealing than those made of metal or cement, for example, wire fences or cement block walls or decks. However, construction of a wood fence or a wood deck is labor intensive. Solid wood components can be heavy and cumbersome. In addition, maintenance of a wood fence or deck is expensive. After a period of time, solid wood fence and deck components will naturally begin to break down from weather exposure and pest infestations. It is known that this deterioration can be tempered by treating the fence or deck with widely available weather resistant coatings, paints, varnishes, finishes and the like. Unfortunately, however, it is often only a matter of time before such treated fences or decks deteriorate requiring partial or complete replacement. Many solid wood materials that are suitable for fencing or decking are costly. In addition, because of natural variations in wood, replacement of individual components may result in an inconsistent, uneven appearance to the fence or deck.
Many products, technologies and ideas have been used to make extruded or molded thermoplastics as an alternative to wood in semi-structural outdoor applications such as decking, park walkways, children's playgrounds, seats and benches. The thermoplastic most widely used is polyethylene, typically a recycled product from HDPE, LDPE & LLDPE milk bottles, film etc. Other thermoplastics widely used include polystyrene, impact modified polystyrene, PVC and polypropylene. Many systems also use a cellulosic filler, typically wood or other natural fibers, compounded into the thermoplastic to enhance properties and make the compound look more like the wooden planks it replaces. These systems are rapidly gaining market acceptance, especially in decks where they have advantages of long-term durability and lack of maintenance. They have an additional advantage because of recent health concerns regarding the chemicals and preservatives used to treat wood for outdoor applications.
Many composites, such as cellulosic/polymer composites, are used as replacements for all-natural wood, particleboard, wafer board, and other similar material. For example, U.S. Pat. Nos. 3,908,902; 4,091,153; 4,686,251; 4,708,623; 5,002,713; 5,087,400; 5,151,238; 5,417,904; 5,948,524; 6,280,667; 6,827,995 and 6,936,200 relate to processes for making plastic/cellulose wood replacement products.
Solid composites of styrene-maleic anhydride and wood based fillers are disclosed in U.S. Pat. Nos. 3,765,934; 3,894,975 and 3,958,069; Canadian Published Patent Application No. 2,626,992 A1 and “Properties of Styrene-Maleic Anhydride Copolymers Containing Wood-Based Fillers”, Simonsen et al., Forest Products Journal, Vol. 48, No. 1, pp. 89-92, January, 1998.
As compared to natural woods, cellulosic/polymer composites offer superior resistance to wear and tear. In particular, cellulosic/polymer composites have enhanced resistance to moisture. In fact, it is well known that the retention of moisture is a primary cause of the warping, splintering, and discoloration of natural woods. Moreover, cellulosic/polymer composites have the appearance of natural wood, and they may be sawed, sanded, shaped, turned, fastened, and finished in the same manner as natural woods. Consequently, cellulosic/polymer composites are commonly used for applications such as interior and exterior decorative house moldings, picture frames, furniture, porch decks, deck railings, window moldings, window components, door components, roofing structures, building siding, and other suitable indoor and outdoor components.
Those skilled in the art have recognized that excessive moisture content in a synthetic wood composition may result in a poor quality end product. In particular, excessive moisture content in a synthetic wood composition may result in an end component that is susceptible to cracking, blistering, and deteriorating appearance. Consequently, it may be necessary to dry any cellulosic material to a predetermined level prior to introducing it into the synthetic wood composition. Even after the cellulosic material is dried, it has a natural tendency to reabsorb moisture from the environment. As a result, it may also be necessary to store the dried cellulosic material in a moisture controlled environment in order to prevent the cellulosic material from reabsorbing additional moisture before being added to the synthetic wood composition. In light of these considerations, it may be difficult and costly to maintain sufficiently dry cellulosic material while shipping it between different locations.
Plastic fence components have been developed as alternatives or supplements to traditional, natural wood fences. For example, U.S. Pat. No. 5,100,109 describes a method of constructing a fence by providing a flexible, plastic, rollable fence board that can be unrolled and fastened to spaced apart fence posts. The flexible fence board is made with height and width dimensions simulating a standard wooden board and with a length of 350 feet or more. According to this patent, the fence board is formed in a continuous extrusion process of a flexible thermoplastic material.
U.S. Pat. No. 5,404,685, describes a wall or fence made in part of foamed polystyrene plastic components, more specifically, plastic columns and panels. Construction of a fence in accordance with this patent requires multiple steps. For example, wall or fence stability is achieved by pouring a reinforcing filler material, such as concrete, into a hollow of the polystyrene plastic columns after the columns have been secured to the ground. A hardened outer surface of the fence is achieved by applying an exterior finish, such as stucco or special exterior paint, to the fence or wall after the fence has been constructed.
However, the synthetic wood or wood composite products described above, typically have disadvantages when their mechanical properties, especially strength and stiffness are compared with the wood they replace. Further, the wood/cellulosic composites described above are susceptible to creep when subjected to continuous loads and/or high ambient temperatures. Additionally, these materials tend to warp after long term exposure to heat. Because of these structural limitations the use of the synthetic wood products described above is often restricted to less structural applications. For example, in decks they are used for deck boards but typically cannot be used for the vertical posts and joists that bear the loads of the whole structure.
Additionally, many of the synthetic wood products described above have densities greater than water, which makes them unfavorable materials to use in marine and related applications.
Thus, there is a need in the art to provide polymer-wood composite products that overcome the above-described problems, as well as methods of making such polymer-wood composite materials.